PU.1 is an ETS family transcription factor that is crucial for the development of multiple hematopoietic lineages including macrophages, B cells, mast cells and neutrophils. In the previous grant period we demonstrated that PU.1 is also expressed in T cells, specifically in Th2 cells. PU.1 expression segregates into Th2 populations secreting low levels of IL-4, but promotes the expression of chemokines associated with allergic inflammation. PU.1-deficient Th2 cells have increased Th2 cytokine production and increased homogeneity of cytokine secretion, in that there are more cells secreting two or more cytokines. Thus, PU.1 regulates heterogeneity in Th2 populations. In the last granting period we have also demonstrated that PU.1 is required for IL-9 expression in the recently described Th9 population of Th cells in both mouse and human cultures, and that PU.1 is expressed at higher levels in Th9 than in Th2 cells. We further show that the development of allergic inflammation in mice that have a T cell specific deletion of PU.1 is decreased, correlating with decreased expression of IL-9 and Th2 chemokines. This demonstrates a requirement for PU.1 in the development of allergic inflammation, a process that was thought to depend largely on Th2 cells. In this proposal, we will further explore the role of PU.1 in Th9 cells, both in the context of gene regulation of IL-9, the development of the Th9 phenotype, and the requirement for PU.1-dependent Th9 cells in the development of allergic inflammation. Our hypothesis for this proposal is that PU.1 is an important regulator of type 2 inflammation and that PU.1-depnedent regulation of the Th2/Th9 phenotypes is a critical component of developing allergic inflammation. Our Aims for this proposal are 1. Define the requirement for PU.1 in mediating allergic inflammation in induced and spontaneous model systems; 2. Determine the structural and functional requirements for the ability of PU.1 to regulate IL-9; and 3. Define the requirement for PU.1 in plasticity and stability of the IL-9-secreting T cell phenotype. Our overall goal for this application is to define the role of Th9 cells in allergic inflammation, and the role of PU.1 in directing this phenotype. The information learned from these studies will provide a greater understanding of the role this subset plays in allergic inflammation, and how targeting this subset, or its functions, might be developed as therapy for allergic disease in humans.